Ofdm modem system

ABSTRACT

A radio relay system ( 1 ) comprises a wireless camera ( 11 ) and a signal receiving relay station ( 12 ). The wireless camera ( 11 ) wirelessly transmits signals to the signal receiving relay station ( 12 ) by using the OFDM modulation method. The wireless camera ( 11 ) and the signal receiving relay station ( 12 ) perform energy dispersion at the time of transmission-line-coding/decoding a transport stream. The PRBS seed (initial value) to be used for the energy dispersion can be externally modified and the user can arbitrarily select a value for the seed.

TECHNICAL FIELD

[0001] This invention relates to an OFDM modulator, an OFDM demodulatorand an OFDM transmission/reception system that can find applications indigital broadcasting such as broadcasting relay facilities usingorthogonal frequency division multiplexing (OFDM) modulation syem.

BACKGROUND ART

[0002] In recent years, many proposals have been made for orthogonalfrequency division multiplexing (OFDM) which is a modulation techniquefor transmitting digital signals. With OFDM modulation, a large numberof orthogonal subcarriers are provided in a transmission band and dataare allocated to the amplitude and the phase of each subcarrier fordigital modulation using PSK (Phase Shift Keying) or QAM (QuadratureAmplitude Modulation). With OFDM modulation, since a transmission bandis divided by a large number of subcarriers, the bandwidth of eachsubcarrier is rather small to make the modulation rate low, although theoverall transmission rate is comparable to that of any known modulationmethod. Additionally, with OFDM modulation, the symbol transmission rateis low because a large number of subcarriers are transmitted inparallel. Because of these characteristics, it is possible with OFDMmodulation to reduce the relative multipath time length relative to thesymbol time length to make the transmission less vulnerable to multipathinterference. Still additionally, with OFDM modulation, since data areallocated to a plurality of subcarriers, the transmission/receptioncircuit can be formed by using an inverse fast Fourier transform (IFFT)operation circuit for modulation and a fast Fourier transform (FFT)operation circuit for demodulation.

[0003] In view of the above identified characteristics of OFDMmodulation, efforts are being paid to apply it to ground wave digitalbroadcasting and communication that are apt to be strongly affected bymultipath interference.

[0004] More specifically, standards for terrestrial digital broadcastingemploying OFDM modulation have been proposed, including the DVB-T(Digital Video Broadcasting-Terrestrial) Standard and the ISDB-T(Integrated Services Digital Broadcasting-Terrestrial) Standard.

[0005] With OFDM modulation, signals are transmitted on the basis of aunit of symbol referred to as OFDM symbol as shown in FIG. 1 of theaccompanying drawings. For signal transmission, an OFDM symbol is madeto comprise an effective symbol covering a signal period good for IFFTand a guard interval formed by copying a rear part of the effectivesymbol. The guard interval is arranged at the head of the OFDM symbol.According to the DVB-T Standard (2K mode), for instance, the effectivesymbol contains 2,048 subcarriers arranged with regular intervals of4.14 kHz. Data are modulated on 1,705 subcarriers out of the 2,048subcarriers in an effective symbol. The guard interval has a time lengthequal to ¼ of that of the effective symbol.

[0006] Firstly, a known OFDM modulator will be described below.

[0007] Referring to FIG. 2 of the accompanying drawings, the known OFDMmodulator 101 comprises a MUX adaptation/energy dispersion circuit 102,a Reed-Solomon encoder 103, a convolutional interleave circuit 104, aconvolutional encoder 105, a bit/symbol interleave circuit 106, amapping circuit 107, a frame adaptation circuit 108, an IFFT circuit109, a guard interval adding circuit 110, a D/A converter 111, a frondend 112, an antenna 113 and a TPS generation circuit 114.

[0008] The OFDM modulator 101 receives as input an MPEG-2 transportstream formed by compressing and multiplexing video and audio signals bymeans of an upstream MPEG encoder. The transport stream is supplied tothe MUX adaptation/energy dispersion circuit 102 of the OFDM modulator101.

[0009] The MUX adaptation/energy dispersion circuit 102 bit-inverts thesyncbyte 47h that is the leading byte of TS packet once in every eightTS packets to turn it to B8h. At this time, it initializes the shiftregister for generating a pseudo-random number series (PRBS) that isused for energy dispersion once in every eight TS packets by using aseed. According to the DVB-T Standard, the PRBS is (x¹⁵+x¹⁴+1) and theseed is 009 Ah. The MUX adaptation/energy dispersion circuit 102operates for energy dispersion by performing an exclusive OR operationof the data excluding the sync byte (1 byte) of the TS packet and thePRBS. The data series that has been subjected to energy dispersion issupplied to the Reed-Solomon encoder 103.

[0010] The Reed-Solomon encoder 103 performs a Reed-Solomon codingoperation on the input data series and adds a parity of 16 bytes foreach TS packet. The data series to which a parity is added is suppliedto the convolutional interleave circuit 104.

[0011] The convolutional interleave circuit 104 performs an interleavingoperation on the input data series. For example, the convolutionalinterleave circuit 104 has 12 branches that are provided with respectivedelay elements having respective amounts of delay that are differentfrom each other as shown in FIG. 3. It selects a same branch for bothinput and output, switching branches for every byte sequentially in amanner such as 0, 1, 2, 3, 4, . . . , 10, 11, 0, 1, 2, . . . It outputsa byte for input a byte and performs a convolutional interleavingoperation. The data series subjected to the convolutional interleavingoperation is then fed to the convolutional encoder 105.

[0012] The convolutional encoder 105 performs convolutional coding bymeans of two encoders such as G1=171 (Octal) and G2=133 (Octal) andoutputs encoded 2 bits for a 1-bit input. When it performs a puncturingoperation, it does so on the output of encoded 2 bits. The data seriessubjected to convolutional coding is then fed to the bit/symbolinterleave circuit 106.

[0013] The bit/symbol interleave circuit 106 interleaves the frequencyin the OFDM symbol and the bits allocated to mapping points. Theinterleaved data series is then supplied to the mapping circuit 107.

[0014] The mapping circuit 107 divides the data series by a code lengthconforming to the employed modulation method (e. g., a code length of 6bits for 64 QAM) and allocates the divided data series to predeterminedmapping points. As a result of allocating the data series to the mappingpoints, two-dimensional information comprising I and Q components isoutput. The data series that is turned to two-dimensional information issupplied to the frame adaptation circuit 108.

[0015] The frame adaptation circuit 108 performs a so-called OFDMframing operation of inserting a predetermined pilot signal, atransmission line multiplexing control signal (TPS: TransmissionParameter Signalling) and a null signal fed from the TPS generationcircuit 114 into the mapped two-dimensional information. The data seriessubjected to OFDM framing is then fed to the IFFT circuit 109.

[0016] The IFFT circuit 109 turns the 2,048 sets of data for I and Q toan OFDM symbol and performs an IFFT operation collectively on it. Thedata series subjected to an IFFT operation is then supplied to the guardinterval adding circuit 110 on an effective symbol by effective symbolbasis.

[0017] The guard interval adding circuit 110 makes a copy of the signalwaveform of the rear ¼ of the signal of each effective symbol outputfrom the IFFT circuit 109 and adds the copy to the head of the effectivesymbol to make it a guard interval. The data series now added with aguard interval is then supplied to the D/A converter 111.

[0018] The D/A converter 111 converts the digital signal into an analogsignal and supplies the latter to the front end 112.

[0019] The front end 112 up-converts the frequency of the analog signalobtained by the D/A conversion to the RF band and transmits it into airby way of the antenna 113.

[0020] Now, a known OFDM demodulator will be described below byreferring to FIG. 4 of the accompanying drawings.

[0021] As shown in FIG. 4, the known OFDM demodulator 131 comprises anantenna 132, a tuner 133, an A/D converter 134, a digital orthogonaldemodulation circuit 135, an FFT operation circuit 136, a narrow band fcerror computation (FAFC) circuit 137, a broad band fc error computationcircuit 138, a numerical-controlled oscillation (NCO) circuit 139, anequalizer 140, a demapping circuit 141, a TPS (Transmission ParameterSignalling) demodulation circuit 142, a bit/symbol deinterleave circuit143, a Viterbi decoding circuit 144, a convolutional deinterleavecircuit 145, a Reed-Solomon decoding circuit 146 and a MUXadaptation/energy inverse dispersion circuit 147.

[0022] The wave transmitted from the broadcasting station for digitaltelevision broadcasting is received by the antenna 132 of the OFDMdecoder 131 and fed to the tuner 133 as RF signal.

[0023] The tuner 133 transforms the frequency of the RF signal receivedby the antenna 132 and outputs an IF signal. The output IF signal isthen fed to the A/D converter 134.

[0024] The A/D converter 134 digitizes the IF signal. The digitized IFsignal is then supplied to the digital orthogonal modulation circuit135. According to the DVB-T Standard (2K mode), the A/D converter 134quantizes the effective symbol and the guard interval of a so-calledOFDM time region signal with a double clock typically for samplingrespectively 4,096 samples and 1,024 samples.

[0025] The digital orthogonal demodulation circuit 135 performsorthogonal demodulation on the digitized IF signal, using the carriersignal of a predetermined frequency (carrier frequency) and outputs abase band OFDM signal. The base band OFDM signal output from the digitalorthogonal demodulation circuit 135 is a so-called time region signalthat is to be subjected to an FFT operation. Therefore, a base bandsignal that has been subjected to digital orthogonal demodulation andyet is to be subjected to an FFT operation is referred to as OFDM timeregion signal hereinafter. As a result of orthogonal demodulation, theOFDM time region signal becomes a complex signal containing a real axiscomponent (I channel signal) and an imaginary axis component (Q channelsignal).

[0026] The OFDM time region signal output from the digital orthogonaldemodulation circuit 135 is then supplied to the FFT operation circuit136 and the narrow band fc error computation circuit 137.

[0027] The FFT operation circuit 136 performs an FFT operation on theOFDM time region signal and extracts the orthogonal-modulated data oneach subcarrier, which data is then output. The signal output from theFFT operation circuit 136 is a so-called frequency region signal thathas been subjected to FFT. Therefore, a signal that has been subjectedto an FFT operation is referred to as OFDM frequency region signalhereinafter.

[0028] The FFT operation circuit 136 extracts the signals in aneffective symbol length (e. g., 2,048 samples) out of an OFDM symbol. Inother words, it extracts signals from the part of an OFDM symbolobtained by excluding the guard interval. Then, it performs an FFToperation on the OFDM time region signal of the extracted 2,048 samples.More specifically, the operation starting position will be found betweenthe boundary of the OFDM symbol (position at A in FIG. 1) and the endposition of the guard interval (position at B in FIG. 1). This range ofoperation is referred to as FFT window.

[0029] Like the OFDM time region signal, the OFDM frequency regionsignal output from the FFT operation circuit 136 is a complex signalcontaining a real axis component (I channel signal) and an imaginaryaxis component (Q channel signal). The OFDM frequency region signal isthen supplied to the broad band fc error computation circuit 138 and theequalizer 140.

[0030] The narrow band fc error computation circuit 137 computes thecarrier frequency error contained in the OFDM time region signal. Morespecifically, the narrow band fc error computation circuit 137 computesthe narrow band carrier frequency error with an accuracy of ±½ of thesubcarrier frequency interval (4.14 kHz) or less. The carrier frequencyerror is the error of the central frequency position of the OFDM timeregion signal that can be produced typically by displacement of thereference frequency output from the local oscillator of the tuner 133.The error rate of the output data increases when this error becomeslarge. The narrow band carrier frequency error determined by the narrowband fc error computation circuit 137 is then fed to the NCO 139.

[0031] The broad band fc error computation circuit 138 computes thecarrier frequency error contained in the OFDM time region signal. Morespecifically, the broad band fc error computation circuit 138 computesthe broad band carrier frequency error with an accuracy of thesubcarrier frequency interval (4.14 kHz) or less. The broad band fcerror computation circuit 138 refers to a continual pilot signal (CPsignal) and computationally determines the extent, or the amount ofshift, by which the CP signal is shifted from the proper insertionposition of the CP signal. The broad band carrier frequency errordetermined by the broad band fc error computation circuit 138 issupplied to the NCO 139.

[0032] The NCO 139 adds the narrow band carrier frequency error of theaccuracy of ±½ of the subcarrier frequency interval as determined by thenarrow band fc error computation circuit 137 and the broad band carrierfrequency error of the accuracy of the subcarrier frequency interval asdetermined by the broad band fc error computation circuit 138 andoutputs a carrier frequency error correction signal whose frequencyincreases/decreases as a function of the carrier frequency errorobtained as a result of the addition. The carrier frequency errorcorrection signal is a complex signal and supplied to the digitalorthogonal demodulation circuit 135. The digital orthogonal demodulationcircuit 135 performs digital orthogonal demodulation, correcting thecarrier frequency fc according to the carrier frequency error correctionsignal.

[0033] The equalizer 140 equalizes the phase and the amplitude of theOFDM frequency region signal, using a scattered pilot signal (SPsignal). The OFDM frequency region signal whose phase and amplitude areequalized is then supplied to the demapping circuit 141 and the TPSdemodulation circuit 142.

[0034] The TPS demodulation circuit 142 separates the TPS signalassigned to a predetermined frequency component and demodulates theinformation containing the coding ratio, the modulation method, theguard interval length and so on from the signal.

[0035] The demapping circuit 141 performs a demapping operation on theOFDM frequency region signal whose phase and amplitude have beenequalized by the equalizer 140 according to the modulation method todecode the data. The demapped data is then fed to the bit/symboldeinterleave circuit 143.

[0036] The bit/symbol deinterleave circuit 143 performs an operationexactly opposite to that of bit-interleaving and symbol-interleavingconducted by the modulator. The data that is subjected tobit-deinterleaving and symbol-deinterleaving is then supplied to theViterbi decoding circuit 144.

[0037] The Viterbi decoding circuit 144 performs a maximum likelihooddecoding operation, using the Viterbi algorithm. The data subjected tomaximum likelihood decoding is then supplied to the convolutionaldeinterleave circuit 145.

[0038] The convolutional deinterleave circuit 145 operates oppositelyrelative to the convolutional interleave circuit of the modulator. Thedata subjected to convolutional deinterleaving is then fed to theReed-Solomon decoding circuit 146.

[0039] The Reed-Solomon decoding circuit 146 decodes the Reed-Solomoncode according to the parity of the 16 bytes added by the modulator andcorrects errors, if any. The data subjected to Reed-Solomon decoding isthen fed to the MUX adaptation/energy inverse dispersion circuit 147.

[0040] If the sync byte of the TS packet that is the leading byte is47h, the MUX adaptation/energy inverse dispersion circuit 147 doesnothing on it. However, if the sync byte is B8h, it inverts the bits andmodifies the byte to 47h. At this time, the MUX adaptation/energyinverse dispersion circuit 147 initializes the shift register forgenerating a pseudo-random number series (PRBS) that is used for energydispersion at every TS packet whose sync byte is B8h by means of apredetermined seed. According to the DVB-T Standard, the PRBS is(x¹⁵+x¹⁴+1) and the seed is 009 Ah. The MUX adaptation/energy inversedispersion circuit 147 operates for energy inverse dispersion byperforming an exclusive OR operation of the data excluding the sync byte(1 byte) of the TS packet and the PRBS. The data series that has beensubjected to energy inverse dispersion is supplied typically to adownstream MPEG-2 decoder as transport stream.

[0041] Meanwhile, wireless cameras are being used for live new reports,live sports coverages and live coverages of various events of televisionbroadcasting. Wireless television cameras provide advantages over cabledtelevision cameras including non-need of cabling and de-cablingoperations and freedom of selection of camera angles and shootingpositions to improve the mobility of cameras on site because signals ofthe images and the sounds taken up by the cameras are transmittedwirelessly by means of ground waves to the base station that may be anoutside broadcast van.

[0042] Additionally, the video signals and audio signals obtained by theshooting operation of the wireless camera are digitized and transmittedto the base station by using a digital modulation method.

[0043] However, a plurality of broadcasting organizations may reportindependently from a site. If the signals are transmitted wirelessly insuch a situation, they may be received not only by the staff of thereporting broadcasting organization but also by the third party that maybe the staff of other broadcasting organizations. The signals mayinclude those of the picked up raw images and sounds as well as those ofauxiliary information.

DISCLOSURE OF THE INVENTION

[0044] In view of the above identified circumstances, it is thereforethe object of the present invention to provide an OFDM modulator, anOFDM demodulator and an OFDM transmission/reception system that canprevent signals including those of picked up images and sounds and thoseof auxiliary information from being received by the third party in asimple way.

[0045] In an aspect of the present invention, there is provided an OFDMmodulator for performing orthogonal frequency division multiplexing(OFDM) modulation on a digital data series comprising an energydispersion means for dispersing energy for the digital data series bymeans of a pseudo-random number series, and an initial value selectingmeans for making the initial value of said pseudo-random number seriesvariable.

[0046] With an OFDM modulator according to the invention, the initialvalue of the pseudo-random number series to be used for energydispersion is varied typically according to the external input.

[0047] In another aspect of the present invention, there is provided anOFDM demodulator for demodulating the digital data series from anorthogonal frequency division mulitplexed (OFDM) signal comprising anenergy inverse dispersion means for performing energy inverse dispersionfor the demodulated digital data series by means of a pseudo-randomnumber series, and an initial value selecting means for making theinitial value of said pseudo-random number series variable according tothe external input.

[0048] With an OFDM demodulator according to the invention, the initialvalue of the pseudo-random number series to be used for energydispersion is varied typically according to the external input.

[0049] In still another aspect of the invention, there is provided anOFDM transmission/reception system for radio transmission of anorthogonal frequency division multiplexed (OFDM) signal comprising atransmitter having an energy dispersion means for dispersing energy foran digital data series by means of a pseudo-random number series, aninitial value selection means for making the initial value of saidpseudo-random number series variable, a modulation means for performingorthogonal frequency division multiplexing (OFDM) modulation for theenergy inverse-dispersed digital dat series and a transmission means forwirelessly transmitting the OFDM signal generated by the OFDMmodulation, and a receiver having a reception means for receiving saidOFDM signal wirelessly transmitted from said transmitter, amodulation/demodulation means for performing orthogonal frequencydivision multiplexing (OFDM) demodulation for the received OFDM signal,an energy inverse dispersion means for inversely dispersing energy forthe demodulated digital data series by means of a pseudo-random numberseries and an initial value selection means for making the initial valueof said pseudo-random number series variable, a same value beingselected for the initial value of the pseudo-random number series ofsaid transmitter and for the initial value of the pseudo-random numberseries of said receiver.

[0050] With an OFDM transmission/reception system according to theinvention, the initial value of the pseudo-random number series to beused by the transmitter and the receiver for energy dispersion is madevariable typically according to the external input.

BRIEF DESCRIPTION OF THE DRAWINGS

[0051]FIG. 1 is a schematic illustration of the signal structure of anOFDM symbol.

[0052]FIG. 2 is a schematic block diagram of a known OFDM modulator.

[0053]FIG. 3 is a schematic illustration of the configuration of aconvolutional interleave circuit.

[0054]FIG. 4 is a schematic block diagram of a known OFDM demodulator.

[0055]FIG. 5 is a schematic illustration of the configuration of a radiorelay system realized by applying the present invention.

[0056]FIG. 6 is a schematic block diagram of a wireless camera that canbe used for the radio relay system of FIG. 5.

[0057]FIG. 7 is a schematic block diagram of the reception relay stationof the radio relay system of FIG. 5.

[0058]FIG. 8 is a schematic block diagram of the OFDM modulator of thewireless camera of FIG. 6.

[0059]FIG. 9 is a schematic circuit diagram of the energy dispersioncircuit of the OFDM modulator of FIG. 8.

[0060]FIG. 10 is a schematic block diagram of the OFDM demodulator ofthe reception relay station of FIG. 7.

BEST MODE FOR CARRYING OUT THE INVENTION

[0061] Now, the present invention will be described by referring to theaccompanying drawings that illustrate a preferred embodiment of theinvention, which is a terrestrial digital radio relay system (to bereferred to as radio relay system hereinafter) that can suitably be usedfor shooting operations of cameras on the site of live coverage of anews report, sports or some other event.

[0062]FIG. 5 is a schematic illustration of the configuration of theembodiment of radio relay system according to the invention.

[0063] Referring to FIG. 5, the radio relay system 1 comprises awireless camera 11, a signal receiving radio relay station 12 forreceiving the signals transmitted from the wireless camera 11.

[0064] The radio relay system 1 is typically used for shootingoperations of the camera on the site of live coverage of a news report,sports or some other event. The video signal of raw image and soundpicked up by the wireless camera 11 is transmitted to the signalreceiving radio relay station 12 by ground wave radio transmission. Itwill be appreciated that the mobility of the camera of the radio relaysystem 1 is remarkably improved on the site of live coverage because thecamera angle and the shooting position of the camera is not constrainedby the cable connecting the camera and the relay station.

[0065] This radio relay system 1 uses transport streams as defined inthe MPEG-2 Systems for radio transmission signals to be transmitted fromthe wireless camera 11 to the radio relay station 12 and adopts the OFDM(orthogonal frequency division multiplexing) modulation method. Thus,the radio relay system 1 can transmit high quality images and soundswith an excellent S/N ratio if compared with a system adapted totransmit analog raw images because it uses transport streams obtained bydigitizing raw images. The OFDM modulation method is characterized inthat it scarcely shows image quality degradation that can be caused byfluctuations in the electric field intensity due to moving signalreception and is hardly influenced by multipath interference. Therefore,it is possible to transmit high quality images and sounds by using theOFDM modulation method.

[0066] Additionally, with the radio relay system 1, it is possible toexternally and arbitrarily select a pseudo-random number series (PRBS)seed (initial value) to be used for energy dispersion at the time ofOFDM modulation of the wireless camera 11 and a pseudo-random numberseries (PRBS) seed (initial value) to be used for energy inversedispersion at the time of OFDM demodulation of the signal receivingrelay station 12. With the radio relay system 1, it is so arrangedbetween the wireless camera 11 and the signal receiving relay station 12that a same seed (initial value) is selected for both the wirelesscamera 11 and the signal receiving relay station 12.

[0067] Now, the configuration of the wireless camera 11 will bedescribed by referring to FIG. 6.

[0068] As shown in FIG. 6, the wireless camera 11 comprises an imagepickup section 21, an MPEG-2 encoder 22, an OFDM modulator 23, afrequency converter 24, a high frequency amplifier 25 and a transmissionantenna 26.

[0069] The image pickup section 21 by turn comprises, among others, aCCD image sensor, an A/D converter and a camera signal processor. Theimage pickup section 21 is a module adapted to process the electricsignal produced from the image picked up by the CCD image sensor foranalog/digital conversion and transmission timing and convert it into adigital video signal. The digital video signal output from the imagepickup section 21 is supplied to the MPEG-2 encoder 22.

[0070] The MPEG-2 encoder 22 receives the digital video signal (V signalin FIG. 6), the digital audio signal (A signal in FIG. 6) obtained bydigitizing the sound signal representing the sound caught by one or morethan one microphones and a predetermined data signal (D signal in FIG.6) as input and performs an operation of compression coding on thesesignals according to the MPEG-2 Systems. Then, the compressed data aremultiplexed to generate a transport stream as defined in the MPEG-2Systems. The transport stream is constituted by transport packets (TSpackets), each having a fixed length of 188 bytes. Video signals, audiosignals and data signals are described in the payload sections of the TSpackets. The transport stream generated by the MPEG-2 encoder 22 issupplied to the OFDM modulator 23.

[0071] The OFDM modulator 23 performs predeterminedtransmission-line-coding operations including energy dispersion, RSencoding, convolutional interleaving, inner coding, bit interleaving,symbol interleaving, mapping conforming to the modulation method andOFDM framing such as insertion of a predetermined pilot signal and anull signal on the input transport stream. Additionally, the OFDMmodulator 23 performs operations of OFDM modulation on thetransmission-line-coded data stream including carrying out an orthogonaltransform of an IFFT (Inverse Fast Fourier Transform) typically by usingan IQ signal of 2,048 sets of data as a symbol for transforming it intoa time region OFDM signal, adding a guard interval to the time regionOFDM signal by copying a rear part of an effective symbol to a leadingpart of the symbol and carrying out orthogonal modulation on the timeregion OFDM signal, to which the guard interval is added, to generate anIF signal of an intermediate frequency band. The configuration of theOFDM modulator 23 will be described in greater detail hereinafter. TheIF signal output from the OFDM modulator 23 is supplied to the frequencyconverter 24.

[0072] The frequency converter 24 transforms the IF signal into an RFsignal that can be transmitted into air by up-converting the carrierwave frequency of the IF signal. The RF signal is then supplied to thehigh frequency amplifier 25.

[0073] The high frequency amplifier 25 performs high frequencyamplification on the RF signal and transmits it into air from thetransmission antenna 26.

[0074] The signal transmitted from the wireless camera 11 having theabove described configuration is received by the signal receiving relaystation 12.

[0075] Thus, the wireless camera 11 having the above describedconfiguration encodes the picked up raw image into a transport streamand OFDM modulates the transport stream, which is transmitted to thesignal receiving relay station 12 by way of a ground wave.

[0076] Now, the signal receiving relay station 12 will be described byreferring to FIG. 7.

[0077] The signal receiving relay station 12 comprises a transmissionantenna 31, a high frequency amplifier 32, a frequency converter 33 andan OFDM demodulator 34.

[0078] The transmission antenna 31 receives the transmission wave of theRF signal transmitted from the wireless camera 11 and forwards it to thehigh frequency amplifier 32.

[0079] The high frequency amplifier 32 performs high frequencyamplification on the RF signal received by the transmission antenna 31.The high-frequency-amplified RF signal is then supplied to the frequencyconverter 33.

[0080] The frequency converter 33 down-converts thehigh-frequency-amplified RF signal to an IF signal of a predeterminedcarrier wave frequency. The frequency-converted IF signal is thensupplied to the OFDM demodulator 34 of receiving section 16 by way of IFcable 15.

[0081] The OFDM demodulator 34 performs processing operations on theinput IF signal, including channel selection and orthogonaldemodulation. Additionally, the OFDM demodulator 34 performs varioussync operations such as FFT window sync and symbol timing sync, whileperforming processing operations of OFDM demodulation includingorthogonal transform of transforming the signal into a frequency regionOFDM signal, using an FFT (Fast Fourier Transform) for each effectivesymbol, waveform equalizing and mapping in order to demodulate thetransmitted data. Furthermore, the OFDM demodulator 34 performstransmission-line-decoding operations on the transmitted and demodulateddata including symbol deinterleaving, bit deinterleaving, inner codedecoding, convolutional deinterleaving, RS decoding and energy inversedispersion in order to demodulate the transmitted data. In this way, thetransport stream transmitted from the wireless camera 11 is output as aresult of the OFDM demodulation/transmission-line decoding. Theconfiguration of the OFDM demodulator 34 will be described in greaterdetail hereinafter.

[0082] The transport stream output from the OFDM demodulator 34 is thentransmitted to the broadcasting station by means of atransmission-line-modulator and a transmitter (not shown). Then, thetransmitted data is processed further in the broadcasting station beforeit is broadcast to the viewers.

[0083] In this way, the signal receiving relay station 12 having theabove described configuration receives the signal transmitted from thewireless camera 11 by terrestrial radio transmission and outputs thecorresponding transport stream.

[0084] Now, the OFDM modulator 23 of the wireless camera 11 will bedescribed in greater detail by referring to FIG. 8.

[0085] As shown in FIG. 8, the OFDM modulator 23 comprises a MUXadaptation/energy dispersion circuit 42, a Reed-Solomon encoder 43, aconvolutional interleave circuit 44, a convolutional encoder 45, abit/symbol interleave circuit 46, a mapping circuit 47, a frameadaptation circuit 48, an IFFT circuit 49, a guard interval addingcircuit 50, an orthogonal modulator 51, a D/A converter 52 and a TPSgeneration circuit 53.

[0086] An MPEG-2 transport stream produced by compressing andmultiplexing video signals and audio signals are input to the OFDMmodulator 23 from the upstream MPEG-2 encoder 22. The transport streamis then fed to the MUX adaptation/energy dispersion circuit 42 of theOFDM modulator 23.

[0087] The MUX adaptation/energy dispersion circuit 42 performs anoperation of bit inversion on the sync byte 47h (1 byte) at the head ofevery eighth TS packet to turn it to B8h. At this time, it initializesthe shift register for generating a pseudo-random number series (PRBS)that is used for energy dispersion at every eighth TS packet by means ofa predetermined seed. The MUX adaptation/energy inverse dispersioncircuit 42 operates for energy dispersion by performing an exclusive ORoperation of the data excluding the sync byte (1 byte) of the TS packetand the PRBS.

[0088] The MUX adaptation/energy dispersion circuit 42 performs energydispersion by means of an energy dispersion processing circuit as shownin FIG. 9. According to the DVB-T Standard, the pseudo-random numberseries to be used for energy dispersion is expressed by an M series ofdegree 15 of (X¹⁵+X¹⁴+1). Although energy dispersion is conductedaccording to the DVB-T Standard in the above description, it mayalternatively be conducted by means of some other series.

[0089] As shown in FIG. 9, the energy dispersion circuit comprises a15-step shift register 61, a first EX-OR circuit 62, a second EX-ORcircuit 63 and an initial value register 64.

[0090] The 15-step shift register 61 is realized by connecting 15-stepregisters for storing a 1-bit data in series. A clock corresponding to abit of input data is entered to the 15-step shift register 61 at a timeso that the data is transferred on a bit by bit basis from a lower orderbit to a higher order bit. The output of the first EX-OR circuit 62 isfed back and input to the register for the least significant bit (LSB).

[0091] Each of the 15-step shift register 61 is provided with a loadterminal. As a load flag is asserted at the load terminal, the valuecurrently held in the corresponding register of the 15-step shiftregister 61 is cleared and the seed (initial value) stored in theinitial value register 64 is loaded there. The seed is loaded at thetiming of detection of the sync byte (B8h) at the head of the TS packet.

[0092] The first EX-OR circuit 62 performs an EX-OR operation of thedata of the 14-th bit and that of the 15-th bit of the 15-step shiftregister and outputs the outcome of the operation. The signal outputfrom the first EX-OR circuit 62 is used as the pseudo-random numberseries of the M series of degree 15 of (x¹⁵ +x¹⁴+1).

[0093] The second EX-OR circuit 63 performs an EX-OR operation of thepseudo-random number series output from the first EX-OR circuit 62 andthe input data (transport stream) on a bit by bit basis and outputs theoutcome of the operation.

[0094] The initial value register 64 stores the seed (initial value: 15bits) of the pseudo-random number series at the head of each TS packet.

[0095] The value to be stored in the initial value register 64 can beselected externally and arbitrarily and modified. In other words, theuser can arbitrarily modify the initial value.

[0096] However, a same seed (initial value) of pseudo-random numberseries needs to be selected for both the transmitter (wireless camera11) and the receiver (signal receiving relay station 12). When the twosides use different seeds, the receiver cannot restore the datadispersed by the transmitter. Therefore, the third party cannot restorethe ultimate raw image and other data as long as the user of the radiorelay system 1 selects the seed (initial value) of pseudo-random numberseries and keeps it as secret.

[0097] While the degree of the pseudo-random number series is made to beequal to 15 in the above description, a pseudo-random number series ofany other degree may be used for the purpose of the invention.

[0098] The data processed for energy dispersion is then fed to theReed-Solomon encoder 43.

[0099] The Reed-Solomon encoder 43 performs a Reed-Solomon codingoperation on the input data series and adds a parity of 16 bytes foreach TS packet. The data series to which a parity is added is suppliedto the convolutional interleave circuit 44.

[0100] The convolutional interleave circuit 44 performs an interleavingoperation on the input data series. For example, the convolutionalinterleave circuit 44 has 12 branches that are provided with respectivedelay elements having respective amounts of delay that are differentfrom each other as shown in FIG. 3. It selects a same branch for bothinput and output, switching branches for every byte sequentially in amanner such as 0, 1, 2, 3, 4, . . . , 10, 11, 0, 1, 2, . . . It outputsa byte for an input byte and performs a convolutional interleavingoperation. The data series subjected to the convolutional interleavingoperation is then fed to the convolutional encoder 45.

[0101] The convolutional encoder 45 performs convolutional coding bymeans of two encoders such as G1=171 (Octal) and G2=133 (Octal) andoutputs encoded 2 bits for a 1-bit input. When it performs a puncturingoperation, it does so on the output of encoded 2 bits. The data seriessubjected to convolutional encoding is then fed to the bit/symbolinterleave circuit 46.

[0102] The bit/symbol interleave circuit 46 interleaves the frequency inthe OFDM symbol and the bits allocated to mapping points. Theinterleaved data series is then supplied to the mapping circuit 47.

[0103] The mapping circuit 47 divides the data series by a code lengthconforming to the employed modulation method (e. g., a code length of 6bits for 64 QAM) and allocates the divided data series to predeterminedmapping points. As a result of allocating the data series to the mappingpoints, two-dimensional information comprising I and Q components isoutput. The data series that is turned to two-dimensional information issupplied to the frame adaptation circuit 48.

[0104] The frame adaptation circuit 48 performs a so-called OFDM framingoperation of inserting a predetermined pilot signal, a transmission linemultiplexing control signal (FPS: Transmission Parameter Signalling) anda null signal fed from the TPS generation circuit 53 into the mappedtwo-dimensional information. The data series subjected to OFDM framingis then fed to the IFFT circuit 49.

[0105] The IFFT circuit 49 turns the 2,048 sets of data for I and Q toan OFDM symbol and performs an IFFT operation collectively on it. Thedata series subjected to an IFFT operation is then supplied to the guardinterval adding circuit 50 on an effective symbol by effective symbolbasis.

[0106] The guard interval adding circuit 50 makes a copy of the signalwaveform of the rear ¼ of the signal of each effective signal outputfrom the IFFT circuit 49 and adds the copy to the head of the effectivesymbol to make it a guard interval. The data series now added with aguard interval is then supplied to the D/A converter 52.

[0107] The orthogonal modulator 51 generated of the IF signal performsorthogonal modulation an I signal and Q signal. The IF signal is thensupplied to the D/A converter 52.

[0108] The D/A converter 52 converts the digital signal into an analogIF signal.

[0109] The IF signal generated by the OFDM modulator 23 is then suppliedto the downstream frequency converter 24.

[0110] Now, the OFDM demodulator 34 in the signal receiving relaystation 12 will be described below by referring to FIG. 10.

[0111] As shown in FIG. 10, the OFDM demodulator 34 comprises an A/Dconverter 74, a digital orthogonal demodulation circuit 75, an FFToperation circuit 76, a narrow band fc error computation (FAFC) circuit77, a broad band fc error computation circuit 78, a numerical-controlledoscillation (NOC) circuit 79, an equalizer 80, a demapping circuit 81, aTPS (transmission parameter signalling) demodulation circuit 82, abit/symbol deinterleave circuit 83, a Viterbi decoding circuit 84, aconvolutional deinterleave circuit 85, a Reed-Solomon decoding circuit86 and a MUX adaptation/energy inverse dispersion circuit 87.

[0112] The IF signal subjected to frequency conversion in the upstreamfrequency converter 33 is input to the OFDM demodulator 34. Then, the IFsignal is fed to the A/D converter 74 of the OFDM demodulator 34.

[0113] The A/D converter 74 digitizes the IF signal. The digitized IFsignal is then supplied to the digital orthogonal demodulation circuit75. According to the DVB-T Standard (2K mode), the A/D converter 74quantizes the effective symbol and the guard interval of a so-calledOFDM time region signal with a double clock typically for samplingrespectively 4,096 samples and 1,024 samples.

[0114] The digital orthogonal demodulation circuit 75 performsorthogonal demodulation on the digitized IF signal, using the carriersignal of a predetermined frequency (carrier frequency) and outputs abase band OFDM signal. The base band OFDM signal output from the digitalorthogonal demodulation circuit 75 is a so-called time region signalthat is to be subjected to an FFT operation. Therefore, a base bandsignal that has been subjected to digital orthogonal demodulation andyet is to be subjected to an FFT operation is referred to as OFDM timeregion signal hereinafter. As a result of orthogonal demodulation, theOFDM time region signal becomes a complex signal containing a real axiscomponent (I channel signal) and an imaginary axis component (Q channelsignal).

[0115] The OFDM time region signal output from the digital orthogonaldemodulation circuit 75 is then supplied to the FFT operation circuit 76and the narrow band fc error computation circuit 77.

[0116] The FFT operation circuit 76 performs an FFT operation on theOFDM time region signal and extracts the orthogonal-modulated data oneach subcarrier, which data is then output. The signal output from theFFT operation circuit 76 is a so-called frequency region signal that hasbeen subjected to FFT. Therefore, a signal that has been subjected to anFFT operation is referred to as OFDM frequency region signalhereinafter.

[0117] The FFT operation circuit 76 extracts the signals in an effectivesymbol length (e. g., 2,048 samples) out of an OFDM symbol. In otherwords, it extracts signals from the part of an OFDM symbol obtained byexcluding the guard interval. Then, it performs an FFT operation on theOFDM time region signal of the extracted 2,048 samples. Morespecifically, the operation starting position will be found between theboundary of the OFDM symbol (position at A in FIG. 1) and the endposition of the guard interval (position at B in FIG. 1). This range ofoperation is referred to as FFT window.

[0118] Like the OFDM time region signal, the OFDM frequency regionsignal output from the FFT operation circuit 76 is a complex signalcontaining a real axis component (I channel signal) and an imaginaryaxis component (Q channel signal). The OFDM frequency region signal isthen supplied to the broad band fc error computation circuit 78 and theequalizer 80.

[0119] The narrow band fc error computation circuit 77 computes thecarrier frequency error contained in the OFDM time region signal. Morespecifically, the narrow band fc error computation circuit 77 computesthe narrow band carrier frequency error with an accuracy of ±½ of thesubcarrier frequency interval (4.14 kHz) or less. The carrier frequencyerror is the error of the central frequency position of the OFDM timeregion signal that can be produced typically by displacement of thereference frequency output from the local oscillator of the tuner 73.The error rate of the output data increases when this error becomeslarge. The narrow band carrier frequency error determined by the narrowband fc error computation circuit 77 is then fed to the NCO 79.

[0120] The broad band fc error computation circuit 78 computes thecarrier frequency error contained in the OFDM time region signal. Morespecifically, the broad band fc error computation circuit 78 computesthe broad band carrier frequency error with an accuracy of thesubcarrier frequency interval (4.14 kHz) or less. The broad band fcerror computation circuit 78 refers to a continual pilot signal (CPsignal) and computationally determines the extent, or the amount ofshift, by which the CP signal is shifted from the proper insertionposition of the CP signal. The broad band carrier frequency errordetermined by the broad band fc error computation circuit 78 is suppliedto the NCO 79.

[0121] The NCO 79 adds the narrow band carrier frequency error of theaccuracy of ±½ of the subcarrier frequency interval as determined by thenarrow band fc error computation circuit 77 and the broad band carrierfrequency error of the accuracy of the subcarrier frequency interval asdetermined by the broad band fc error computation circuit 78 and outputsa carrier frequency error correction signal whose frequencyincreases/decreases as a function of the carrier frequency errorobtained as a result of the addition. The carrier frequency errorcorrection signal is a complex signal and supplied to the digitalorthogonal demodulation circuit 75. The carrier frequency errorcorrection signal performs digital orthogonal demodulation, correctingthe carrier frequency fc according to it signal.

[0122] The equalizer 80 equalizes the phase and the amplitude of theOFDM frequency region signal, using a scattered pilot signal (SPsignal). The OFDM frequency region signal whose phase and amplitude areequalized is then supplied to the demapping circuit 81 and the TPSdemodulation circuit 82.

[0123] The TPS demodulation circuit 82 separates the TPS signal assignedto a predetermined frequency component and demodulates the informationcontaining the coding ratio, the modulation method, the guard intervallength and so on from the signal.

[0124] The demapping circuit 81 performs a demapping operation on theOFDM frequency region signal whose phase and amplitude have beenequalized by the equalizer 80 according to the modulation method todecode the data. The demapped data is then fed to the bit/symboldeinterleave circuit 83.

[0125] The bit/symbol deinterleave circuit 83 performs an operationexactly opposite to that of bit-interleaving and symbol-interleavingconducted by the OFDM modulator. The data that is subjected tobit-deinterleaving and symbol-deinterleaving is then supplied to theViterbi decoding circuit 84.

[0126] The Viterbi decoding circuit 84 performs a maximum likelihooddecoding operation, using the Viterbi algorithm. The data subjected tomaximum likelihood decoding is then supplied to the convolutionaldeinterleave circuit 85.

[0127] The convolutional deinterleave circuit 85 operates oppositelyrelative to the convolutional interleave circuit of the OFDM modulator.The data subjected to convolutional deinterleaving is then fed to theReed-Solomon decoding circuit 86.

[0128] The Reed-Solomon decoding circuit 86 decodes the Reed-Solomoncode according to the parity of the 16 bytes added by the OFDM modulatorand corrects errors, if any. The data subjected to Reed-Solomon decodingis then fed to the MUX adaptation/energy inverse dispersion circuit 87.

[0129] If the sync byte of the TS packet that is the leading byte is47h, the MUX adaptation/energy inverse dispersion circuit 87 doesnothing on it. However, if the sync byte is B8h, it inverts the bits andmodifies the byte to 47h. At this time, the MUX adaptation/energyinverse dispersion circuit 87 initializes the shift register forgenerating a pseudo-random number series (PRBS) that is used for energydispersion at every TS packet whose sync byte is B8h by means of apredetermined seed. The MUX adaptation/energy inverse dispersion circuit87 operates for energy inverse dispersion by performing an exclusive ORoperation of the data excluding the sync byte (1 byte) of the TS packetand the PRBS.

[0130] The MUX adaptation/energy inverse dispersion circuit 87 has acircuit configuration same as the energy dispersion circuit of thewireless camera 11.

[0131] Thus, with the inverse dispersion circuit, the seed (initialvalue) to be given to the 15-bit shift register is also externally andarbitrarily selected and hence modifiable. Then, a same value isselected for the seed (initial value) of pseudo-random number series forboth the transmitter side (wireless camera 11) and the receiver side(signal receiving relay station 12) so that the raw images and otherdata transmitted from the wireless camera 11 can be restored at thesignal receiving relay station 12.

[0132] The data series subjected to energy inverse dispersion is thenfed to the broadcasting station as transport stream.

[0133] Thus, with the above described embodiment of radio relay system1, the seed (initial value) of the energy dispersion/inverse dispersioncircuit that is also commonly used in known OFDM modulator/demodulatorcan be selected and modified arbitrarily. Therefore, it is possible forthe radio relay system 1 to enhance the confidentiality of raw imagesand sounds and auxiliary data and prevent them from leaking to the thirdparty with a simple arrangement. In other words, only the staff of theradio relay system can use the raw images and sounds.

[0134] While the present invention is described above in terms of aradio relay system of a broadcasting station, the present invention isby no means limited thereto. For example, the present invention is alsoapplicable to any systems that require an enhanced level of security, bethey home use or professional use such as those used in broadcastingstations. Additionally, the transmitter side is not limited to a cameraand may alternatively be any facility adapted to transmit data by awireless transmission path.

[0135] While a same seed (initial value) of pseudo-random number seriesis selected externally by the user in the above description of theembodiment, it may alternatively be so arranged that the transmitterside transmits a signal containing the initial value and the receiverside automatically retrieve and use the initial value. For instance, itmay be so arranged that the TPS added at the time of OFDM framingcontains the initial value. Still alternatively, it may be so arrangedthat the seed (initial value) is selected by some means at either thetransmitter side or the receiver side and transmitted to the other sideby some information transmitting means (e. g., telephone).

[0136] Industrial Applicability

[0137] With an OFDM modulator, an OFDM demodulator and an OFDMtransmission/reception system according to the invention, the initialvalue of pseudorandom number series to be used for energy dispersion canbe selected and modified externally. Therefore, it is possible toenhance the confidentiality of the raw images and sounds and auxiliarydata transmitted wirelessly and prevent them from leaking to the thirdparty with a simple arrangement.

1. An OFDM modulator for performing orthogonal frequency divisionmultiplexing (OFDM) modulation on a digital data series comprising: anenergy dispersion means for dispersing energy for the digital dataseries by means of a pseudo-random number series; and an initial valueselecting means for making the initial value of said pseudo-randomnumber series variable.
 2. The OFDM modulator according to claim 1,further comprising: a means for transmitting information indicating saidinitial value of said pseudo-random number series selected by saidinitial value selected means.
 3. An OFDM demodulator for demodulatingthe digital data series from an orthogonal frequency divisionmulitplexed (OFDM) signal comprising: an energy inverse dispersion meansfor performing energy inverse dispersion for the demodulated digitaldata series by means of a pseudo-random number series; and an initialvalue selecting means for making the initial value of said pseudo-randomnumber series variable according to the external input.
 4. An OFDMdemodulator according to claim 3, further comprising: a restoring meansfor restoring information indicating an initial value of saidpseudo-random number series as transmitted to it; said initial valueselecting means being adapted to select an initial value of saidpseudo-random number series on the basis of the information indicatingsaid initial value as restored by said restoring means.
 5. An OFDMtransmission/reception system for radio transmission of an orthogonalfrequency division multiplexed (OFDM) signal comprising: a transmitterhaving an energy dispersion means for dispersing energy for an digitaldata series by means of a pseudo-random number series, an initial valueselection means for making the initial value of said pseudo-randomnumber series variable, a modulation means for performing orthogonalfrequency division multiplexing (OFDM) modulation for the energyinverse-dispersed digital data series and a transmission means forwirelessly transmitting the OFDM signal generated by the OFDMmodulation; and a receiver having a reception means for receiving saidOFDM signal wirelessly transmitted from said transmitter, amodulation/demodulation means for performing orthogonal frequencydivision multiplexing (OFDM) demodulation for the received OFDM signal,an energy inverse dispersion means for inversely dispersing energy forthe demodulated digital data series by means of a pseudo-random numberseries and an initial value selection means for making the initial valueof said pseudo-random number series variable, a same value beingselected for the initial value of the pseudo-random number series ofsaid transmitter and for the initial value of the pseudo-random numberseries of said receiver.
 6. The OFDM transmission/reception systemaccording to claim 5, wherein said initial value is transmitted betweensaid transmitter and said receiver.